Thermoset molding is normally accomplished by low pressure or high pressure techniques. Low pressure techniques involve various types of casting, reaction injection molding or similar techniques. Castings are usually made by pouring or forcing reactable liquid mixtures into molds and heating until completely polymerized. Reaction injection molding is a process whereby two or more independent reactive liquid ingredients are metered through a mixing head where they are combined by impingement mixing and shot into a mold where they react to form a molded part. These low pressure molding methods tend to produce medium to large sized parts reasonably economically but are not as competitive when mass producing small intricate parts (for example, parts of about 20 grams or less per part).
High pressure thermoset molding involves processes such as thermoset injection molding, compression molding, transfer molding and the like, typically at pressures of the order of 200 to 2000 psi. Thermoset injection molding tends to be the opposite of thermoplastic injection molding. Thermoplastic injection molding is accomplished by melting the polymeric material and forcing it into a water cooled mold for the necessary time to solidify the parts for removal. Thermoset injection molding is accomplished by forcing a crosslinkable liquid organic into a heated mold for a controlled cure time irreversibly cross-linking the material into final parts which are removed hot.
Compression molding has been used for molding thermoset materials including various rubbers for many years and is still in common use today. The main advantages of compression molding is its simplicity. The material containing a suitable curing agent is placed in a heated cavity, the mold is closed and pressurized for the required cure time. Tool costs are relatively low and material is not wasted by formation of sprues and runners (channels) which are not required.
Transfer molding is similar to compression molding except the measured charge of thermoset material is placed into a cavity called a pot. A plunger matched to the pot walls forces the material through sprues and runners passing through a final restriction called a gate into the cavities. Air in the cavities is displaced by the incoming material through the parting line of the mold which allows passage of air but not of the thick viscous liquid. The material is maintained at 280 to 380 degrees F., rapidly curing the parts.
When attempting to make highly energy absorbent polyurethane materials having good mechanical properties, difficulties are often encountered. Frequently, when making such materials the tensile strength is low. Additionally, these liquid or low temperature meltable materials are too low in viscosity at the higher processing temperatures (about 300 degrees F.) for the preferred high pressure molding systems. High pressure systems, especially transfer molding, are really needed to produce small intricate parts very economically. The problem comes when attempting to mass produce large volumes of thermoset parts from liquid components using high pressure molding, and especially transfer molding. The low viscosity of the liquids allows them to flow through the mold parting line under the high pressure. As a result, the liquid leaks out and the pressure goes to zero resulting in incomplete parts.